1. Field Of The Invention
This invention relates to arc discharge lamps and, in particular, to high pressure sodium vapor lamps.
2. Description of the Prior Art
Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps is of the order of about 50- 100 millimeters Hg.
Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in order of a few microns. Low pressure sodium lamps have been in use for about 30 or 40 years, but, although efficient, they produce an unattractive monochromatic yellow light. The color of light from high pressure sodium lamps is considerably improved over that from low pressure sodium lamps.
High pressure sodium lamps generally comprise an alumina ceramic arc tube and an arc tube fill of sodium, mercury, and an inert gas. Examples of such lamps are shown in the following U.S. Pat. Nos.: 3,248,590; 3,363,113; 3,384,798; 3,385,463; 3,428,846; 3,448,319; 3,453,477; 3,474,277; 3,485,343; 3,519,406; 3,520,039; 3,521,108; 3,558,963; 3,604,972; 3,609,437; 3,622,217; 3,650,593; 3,682,525; 3,708,710; 3,716,743; 3,716,744.
One of the problems of such lamps concerns the ceramic to metal seals at the ends of the alumina arc tube. Generally, a double seal is used. For example, a metal end cap is sealed to the end of a uniform diameter arc tube and a metal tube, usually niobium, is sealed to the metal end cap. Such seals are shown in U.S. Pat. Nos. 3,248,590; 3,384,798; 3,448,319; 3,479,170; 3,519,406; 3,588,577; 3,598,435; 3,450,927; 3,474,277; 3,480,823; 3,497,756; 3,682,525; 3,520,039; 3,385,463; 3,428,846; 3,604,972; 3,716,744; 3,721,845; and 3,721,846.
The metal end cap seal has been widely used commercially, but suffers from the very high cost of the metal end caps which are made of nobium metal. In addition, the high temperature glass-like frit which seals the metal end cap to the alumina tube is not capable of operating above about 800.degree. C for thousands of hours and this places an upper limit on the temperature on the seal and within its vicinity.
Another type of end cap widely used, particularly in Europe, is a ceramic end plug disposed in the end of a ceramic arc tube while both the arc tube and end plug are in an unsintered state such that, upon high temperature sintering, the end plug forms a gas tight seal with the arc tube. A secondary end plug can be sealed to this configuration by a high temperature glass-like frit. This art is disclosed in U.S. Pat. Nos. 3,564,328 and 3,609,437. This type of seal is expensive because of the need of closely dimensioning the unsintered end plug to the unsintered arc tube such that on sintering they diffuse together. It is apparent that in both the metal cap construction and the ceramic cap construction, double seals are needed which are both expensive and lend themselves to potential difficulties because of the added processing.
It is an object of this invention to eliminate such double seals and to reduce the ceramic-to-metal seal area in order to provide a more reliable longer-life arc tube.